Fair Value Measurements - Schedule of Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Sep. 30, 2022 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 53,400.0
Fair value	46,400.0
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	10,800.0
Fair value	9,300.0
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	14,500.0
Fair value	12,600.0
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1,500.0
Fair value	1,200.0
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	2,900.0
Fair value	2,700.0
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	7,400.0
Fair value	$ 6,300.0